Call WebFrameWidget::setVisibilityState on RFI::WasShown

The call to WebViewImpl::setVisibilityState updates Blink's Page state
correctly, but was not notifying the WebFrameWidget's LayerTreeHost.

This patch calls a newly added method in WebFrameWidget that resolves
some rendering issues.

BUG=450674

Review URL: https://codereview.chromium.org/958973002

Cr-Commit-Position: refs/heads/master@{#318709}

content/renderer/render_frame_impl.cc

@@ -3615,8 +3615,8 @@ void RenderFrameImpl::WasShown() {
   // VisibilityState remain a page-level concept or move to frames?
   // The semantics of 'Show' might have to change here.
   if (render_widget_) {
-    render_view()->webview()->setVisibilityState(
-        blink::WebPageVisibilityStateVisible, false);
+    static_cast<blink::WebFrameWidget*>(render_widget_->webwidget())->
+        setVisibilityState(blink::WebPageVisibilityStateVisible, false);
   }
   FOR_EACH_OBSERVER(RenderFrameObserver, observers_, WasShown());
 }